1. Field of the Invention
The present invention relates to a piezoelectric resonator and a method of manufacturing a piezoelectric resonator, and relates to a technique of a piezoelectric resonator that is low in cost and has small variations in a device characteristic.
2. Description of the Related Art
A piezoelectric resonator in an SMD (a surface-mount) structure, which is, for example, a crystal resonator (that will be called an “SMD-type crystal resonator”, hereinafter), is small in size lightweight, so that it is housed in a portable electronic device represented as, for example, a cellular phone as a reference source of frequency or time. One example of the above SMD-type crystal resonator will be explained by using FIG. 9(a) and FIG. 9(b). In the drawings, 11 denotes a ceramic base with an upper surface open, and 12 denotes a metal cover, and the base 11 and the cover 12 are seam-welded with a sealing member 13 made of, for example, a welding material, and the inside thereof is in a vacuum state.
In the drawings, 10 denotes a crystal resonating piece, and as the above crystal resonating piece 10, a tuning-fork crystal resonating piece, a rectangular-shaped crystal resonating piece, or the like can be used. The tuning-fork crystal resonating piece 10 will be explained by using FIG. 10. Paired vibrating arm portions 21a and 21b are provided in a base portion 2, and in both main surfaces of the respective vibrating arm portions 21a, 21b, groove portions 23, 24 are provided respectively. In the groove portions 23, 24 and on the respective vibrating arm portions 21a, 21b, a not-illustrated excitation electrode for exciting tuning-fork vibration based on flexural vibration is formed. Further, on a base 11 side of the crystal resonating piece 10, first and second electrode terminals (that are not-illustrated) to be electrically connected to the above-described excitation electrodes respectively are led out to the base 11 side from the excitation electrodes to be provided on the right and left.
The above crystal resonating piece 10 has the electrode terminals on the base portion 2 fixed to a pedestal portion 14 of the base 11 with a conductive adhesive 15 in the posture in which the vibrating arm portions 21a, 21b extend sideways in an inner space formed by the base 11 and the cover 12, and in this manner, the crystal resonating piece 10 is attached to the base 11 substantially horizontally. On the other hand, in the region, of the pedestal portion 14, to which one end side of the crystal resonating piece 10 is attached, conductive paths 16, 17 are formed (17 denotes the conductive path positioned on a far side of the paper). An oscillation operation of the crystal resonating piece 10 formed in this manner is caused when voltage is applied to the crystal resonating piece 10 via electrodes 18, 19 provided on an outer bottom surface of the base 11 in a longitudinal direction so as to face each other, the conductive paths 16, 17, and the conductive adhesive 15.
However, in the SMD-type crystal resonator as above, the region, of the pedestal portion 14, to which the one end side of the crystal resonating piece 10 is attached is substantially the same as other regions in height. Thus, the conductive adhesive 15 easily flows outward, and the conductive adhesive spreads over an area larger than necessary. Accordingly, the region where the conductive adhesive 15 spreads varies in element, and thus there is a problem that variations in terms of a device characteristic easily occur.
In order to solve the above problem, it has been considered that the height of the region, of the base 11, to which the electrode terminals on the crystal resonating piece 10 are attached is increased to be higher than those of the other regions to make it difficult for the conductive adhesive to spread. A structure in which, for example, on the base 11 side, projection-shaped electrodes each made of a conductive material are provided, and the above projection-shaped electrodes and the electrode terminals on the crystal resonating piece 10 are electrically connected with a conductive adhesive has been thought.
The projection-shaped electrodes as above each have been formed in a manner that, in general, for example, a base metal made of tungsten (W) and so on is formed on the base 11 to have a film thickness of 10 μm to 15 μm or so, and then on the base metal, nickel (Ni)/gold (Au) plating is performed. Here, the thickness of the base metal is 10 μm to 15 μm or so, so that a thickness of Au has to be 10 μm to 30 μm or so in order to obtain a height of each of the projection-shaped electrodes, and Au is expensive to thus cause a problem in terms of cost.
In the above case, it is also thought that a metal more inexpensive than Au is used to obtain the height of each of the projection-shaped electrodes, and then on a front surface of the metal, Ni/Au plating is performed. However, in the above method, film forming of the metal for increasing the projection-shaped electrodes in height and film forming of Au have to be performed, thereby increasing the number of processes, resulting that an increase in cost is caused.
However, in Patent Document 1, there has been described a structure in which an independent projecting portion is provided at electrode forming positions on an upper surface of an insulating base. The above example is to achieve a reduction in cost by reducing materials for forming the insulating base, but in practice, it has not been described by which method the projecting portions are manufactured, resulting that the structure in Document 1 also cannot solve the problem of the present invention.
[Patent Document 1] Japanese Patent Application Laid-open No. 2000-164747 (paragraph 0017, FIG. 5)